I've been using rayon in my RM2 and my Cyclone for about six weeks now. I use the RM2 with single coils at around 0.5 ohms and the Cyclone with dual coils at around 0.4 ohms.
I make contact coils on a 2mm (0.08", or 5/64") diameter screw driver, so a bit bigger than regular micro coils at 1.5mm max. I found with cotton the extra diameter helps me avoid dry hits at 25 - 35 watts so I just continued doing that with rayon.
A few days ago I tried organic cotton in the Cyclone again just to see the difference now that I've been using rayon for a while. I went back to rayon after a day. The organic cotton just didn't wick fast enough for me to continue to use it.
PBusardo did a review of hemp, Japanese cotton, organic cotton and rayon and found the organic cotton was the slowest in wicking. Koh Gen Do (Japanese cotton) just about tied with rayon for him if I remember correctly. I have not tried KGD myself, so I have no opinion on it, but from PBusardo's review it's right up there with rayon.